1. Field of the Invention
The present invention relates to an image pickup apparatus, a white balance control method, and a white balance control program.
2. Description of the Related Art
Conventionally, it has been well known that an electronic still camera which is a type of an image pickup apparatus causes a liquid crystal monitor to display an object image picked up by using a solid state image pickup device such as a CCD as a through image (a finder image) regardless of a shutter operation in an imaging wait state in which a predetermined mode (such as a REC mode) is set. In such an electronic still camera, it is necessary to obtain a white balance in order to reproduce a color of an object more precisely. The white balance is provided to whitely display a white object even when an image is picked up under light with different color temperatures. A gain of an image signal output from the solid state image pickup device is controlled for each of color components R, G, and B according to an imaging environment (a light source), thereby making it possible to allocate the white balance. Therefore, it is possible to obtain a white balance close to an optimal state by storing in a memory a gain value which corresponds to expected plural types of imaging environments (light sources) or a correction coefficient of a reference gain is stored in a memory, and by causing a user to set an actual imaging environment (a light source).
However, this forces the user to make a complicated operation, and thus, an auto white balance function for automatically obtaining a white balance is provided. In order to carry out auto white balance, it is necessary to determine a which portion of the picked-up image is white. In order to determine this white portion, first, an output signal (an image signal) of the solid state image pickup device is YUV-converted, and a color difference signal (a Cb signal, a Cr signal) is obtained. By using this color difference signal, color information, namely, a Cb value and a Cr value are checked relevant to all the pixels configuring an image. At this time, when the Cb value and Cr value of a certain pixel are included in a predetermined white detecting range WS (in a range in which there is a high possibility of being white), it is determined that the color of that pixel is white. The Cb value and Cr value of all the pixels determined to be white are integrated. Then, a gain of an R component and a gain of a B component are controlled so that the integrated value of Cb and the integrated value of Cr are “0” (Cb=Cr=0). That is, in a state in which a white becomes slightly blue (when imaging is carried out under light with a high color temperature), the gain of the R component is increased, and the gain of the B component is reduced. In a state in which white becomes slightly red (when imaging is carried out under light with a low color temperature), the gain of the R component is reduced, and the gain of the B component is increased.
In such a method, when an object appearing to be write under sun light (standard light of 5500 K) is imaged under an imaging condition (imaging light) which is different from another one, a range in which that color (a Cb value, a Cr value) changes is set as a white detecting range WS, whereby, even if the imaging condition (imaging light) is different, a proper white balance can be automatically obtained.
In addition, there has been proposed an auto white balance control method in which, a proper white balance can be automatically obtained while in strobe light emission (refer to, for example, Japanese Patent KOKAI Publication 2000-308069). This auto white balance control method includes comparing object brightness obtained by adding a constant indicative of an increment of object brightness during strobe light emission to the object brightness optically measured before image pickup with object brightness optically measured during strobe light emission imaging. As a result of this comparison, when the object brightness optically measured during strobe light emission is greater, it is determined that type of a light source is strobe light, and a white balance is controlled.
However, in the previously described auto white balance control method, when type of the light source is strobe light, a xenon tube which is a strobe light source has a color temperature of 5500 K which is equal to sun light. Thus, the gain setting of the white balance is performed in the same manner as in the case of sun light. Therefore, there is no problem in the case where no external light exists or in the case where external light is sun light. In a state in which there is any external light other than sun light, and this light hits an object, if strobe light is emitted, the picked-up and recorded image is reproduced as an image with the color of external light because the gain setting of the white balance is performed based on sun light.